The electromagnetic problem described by Eq. (REF ) can conveniently be reformulated by introducing the notion of dyadic Green function [1]}. The dyadic Green function \({\bf G}({\bf r},{\bf r}^{\prime })\)  of a medium describes the electric field produced at point \({\bf r}\)  by a radiating point electric dipole at point \({\bf r}^{\prime }\) . While it can be defined for an arbitrary environment, we are interested here in the dyadic Green function in the background medium, \({\bf G}_\text{b}\) , which is then the solution of
\({\nabla } \times {\nabla } \times {\bf G}_\text{b}({\bf r},{\bf r}^{\prime }) - k_\text{b}^2 {\bf G}_\text{b}({\bf r},{\bf r}^{\prime }) = {\bf I}\delta ({\bf r}-{\bf r}^{\prime }). \) 
